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Gravitational Interactions Newton Again • Sir Isaac Newton did not discover gravity. – He discovered that gravity is universal – All matter attracts all other matter. Can you name a few ways gravity affects things on our planet?? Have you ever heard of a force field? • A force field surrounds things with mass. We call it a gravitational field. Remember this picture… What is your acceleration as you fall? a=9.8m/s2 a=4.9m/s2 a=0m/s2 a=4.9m/s2 a=9.8m/s2 What would your weight be if you were in a cavity at the center of the Earth? Your weight would be zero! Imagine a journey to the center of the Earth! What motion would you experience? Weight & Weightlessness • What would a scale read (higher or lower than normal) if your were on an elevator: – Accelerating up? • Higher! – Accelerating down? • Lower! – In freefall after the cable breaks? • Zero! Weight & Weightlessness • Weightlessness is not the absence of gravity. It is the absence of a support force. Why do astronauts feel weightless? Why do astronauts feel weightless? • If you had a ladder that could reach as high as a shuttle's orbit, your weight would be 11% less at the top. • A person who weighs 100 pounds on the Earth's surface would weigh about 89 pounds at the top of the ladder. • The astronaut, the spaceship and everything inside it are falling towards the Earth. • They are always falling towards the Earth, but they never get there. Let’s revisit good old “g” Weight on Earth m=object’s mass on Earth r=Earth’s radius F F mg g m m1m2 FG 2 d mM FG 2 r M=Earth’s mass Let’s revisit good old “g” F g m GM g 2 r mM G 2 r g m The value of “g” GM g 2 r 11 24 (6.67 10 )(5.98 10 kg) g 6 2 (6.37 10 m) g 9.8m/s How can we figure out the 2 acceleration due to gravity on any other planet? The moon’s radius is 1737000m and its mass is 7.36x1022kg. How long would it take for a rock to reach the ground if dropped from a height of 2m? GM g 2 r 1 2 d gt 2 (6.67 1011)(7.36 1022 kg) g 2 1737000 2 g 1.63m/s 2d 2(2m) t 1.57s 2 g 1.63m/s A neutron star has a radius of only 6000m, but its mass is 6x1030kg! How much would you weigh if you were able to stand on its surface?? A neutron star has a radius of 6000m and its mass is 6x1030kg. How much would you weigh if you were able to stand on its surface?? GM g 2 r 11 (6.67 10 )(6 10 kg) g 2 6000 g162,400,000,000,000 1.111013 m/s2 pounds!!!! weight F mg F 65kg(1.1110 m/s ) 13 F 7.22 10 N 14 2 30 OCEAN TIDES • What causes them? – Give your best explanation on a whiteboard. OCEAN TIDES Low Tide High Tide High Tide Low Tide Why are there two per day? Because the Earth spins under this bulge! Largest and smallest tides • Neap tide- sun is at 90 degrees relative to Earth and moon, tide is smallest • Spring tide- Sun, Moon, and Earth are Aligned, tide is largest. Do you know what big is? • Astronomical Scale Our Galaxy There is an ongoing battle between thermonuclear fusion and gravity… Gravity Watch out if gravity wins! Fusion Red Giant- the swelling of a star Supernova • The explosion of a star Supernova Remnant • White Dwarf Star • More mass stars end up as Black Holes Black Holes Black Holes Same gravitational field strength because neither mass nor distance changes. Black Holes: • • • • Alters the space around it Enormously dense Very strong gravitational field Escape velocity is greater than the speed of light hence…BLACK hole How do you think black holes might be discovered?